migratory fauna

The hydrologic, geomorphic, and ecological dynamics of tropical montane
streams are poorly understood in comparison to many temperate and/or alluvial rivers.
Yet as the threat to tropical freshwater environments increases, information on the
dynamics of relatively pristine streams is important for understanding landscape
evolution, managing and conserving natural resources, and implementing stream
restoration. This dissertation characterizes the geomorphology and hydrology of five
adjacent watersheds draining the Luquillo Experimental Forest (LEF) in northeastern
Puerto Rico, and discusses implications on aquatic habitat. I performed several
interrelated studies, including: 1) formulating a geographic information systems (GIS)
framework to estimate hydrologic parameters from topographic information and
hydrologic records, 2) developing a method to determine active stream channel
boundaries (“bankfull” stage) that allows for comparison of channel geometry on the
basis of flow-frequency, 3) decoupling the relative influences of lithologic and hydraulic
controls on channel morphology using an extensive field-based stream survey and
analysis of stream profiles, channel geometry, and sediment dynamics, 4) linking
network- and pool-scale geofluvial dynamics to the abundance of migratory fish and
shrimp through a collaborative analysis combining geomorphic surveys and aquatic
faunal sampling. This research indicates that these streams have some properties
resembling both temperate montane and alluvial rivers. Similar to low-gradient rivers
where floodplains mark channel boundaries, the active channel stage in these streams is
defined by the incipient presence of woody vegetation and soil development. Systematic
basin-scale geomorphic patterns are well-developed despite apparent non-fluvial and
lithologic control on local channel morphology. This implies that strong fluvial forces are
sufficient to override channel boundary resistance; a feature common in self-forming
“threshold” alluvial channels. Migratory aquatic fauna abundances are influenced by a
variety of geomorphic factors such as barrier waterfalls and suitable headwater habitat,
and are consequently highly variable and patchy. These results stand in contrast to the
notion that aquatic communities mirror systematic geomorphic gradients, but rather
acknowledges the influences of multiscale geomorphic processes. Ultimately, this
research provides baseline information on physical and biological processes in relatively
unaltered tropical streams and can be used to inform further studies that document human
interactions with stream networks.

Large dams degrade the integrity of a wide variety of ecosystems, yet direct downstream effects of dams have received the most attention from ecosystem managers and researchers. We investigated indirect upstream effects of dams resulting from decimation of migratory freshwater shrimp and fish populations in Puerto Rico, USA, in both high- and low-gradient streams. In high-gradient streams above large dams, native shrimps and fishes were extremely rare, whereas similar sites without large dams had high abundances of native consumers. Losses of native fauna above dams dramatically altered their basal food resources and assemblages of invertebrate competitors and prey. Compared to pools in high-gradient streams with no large dams, pool epilithon above dams had nine times more algal biomass, 20 times more fine benthic organic matter (FBOM), 65 times more fine benthic inorganic matter (FBIM), 28 times more carbon, 19 times more nitrogen, and four times more non-decapod invertebrate biomass. High-gradient riffles upstream from large dams had five times more FBIM than did undammed riffles but showed no difference in algal abundance, FBOM, or non-decapod invertebrate biomass. For epilithon of lowgradient streams, differences in basal resources between pools above large dams vs. without large dams were considerably smaller in magnitude than those observed for pools in highgradient sites. These results match previous stream experiments in which the strength of native shrimp and fish effects increased with stream gradient. Our results demonstrate that dams can indirectly affect upstream free-flowing reaches by eliminating strong top-down effects of consumers. Migratory omnivorous shrimps and fishes occur throughout the tropics, and the consequences of their declines upstream from many tropical dams are likely to be similar to those in Puerto Rico. Thus, ecological effects of migratory fauna loss upstream from dams encompass a wider variety of species interactions and biomes than the bottom-up effects (i.e., elimination of salmonid nutrient subsidies) recognized for northern temperate systems.

1. Compared to most other tropical regions, Puerto Rico appears to have dammed its
running waters decades earlier and to a greater degree. The island has more large dams per unit
area than many countries in both tropical and temperate regions (e.g., 3x that of the U.S.), and
the peak rate of large dam construction occurred two and three decades prior to reported peak
rates in Latin America, Asia and Africa.
2. Puerto Rico is a potential window into the future of freshwater migratory fauna in
tropical regions, given the island’s extent and magnitude of dam development and the available
scientific information on ecology and management of the island’s migratory fauna.
3. We review ecology, management and conservation of migratory fauna in relation to
dams in Puerto Rico. Our review includes a synthesis of recent and unpublished observations on
upstream effects of large dams on migratory fauna and an analysis of patterns in free crest
spillway discharge across Puerto Rican reservoirs. Analyses suggest that large dams with rare
spillway discharge cause near, not complete, extirpation of upstream populations of migratory fauna. They also suggest several management and conservation issues in need of further
research and consideration. These include research on the costs, benefits and effectiveness of
simple fish/shrimp passage designs involving simulating spillway discharge and the
appropriateness of establishing predatory fishes in reservoirs of historically fishless drainages.